It is known that polyisoprenes having a high cis-1,4 linkage content may be prepared using catalytic systems based on:                a rare earth salt in solution in a hydrocarbon solvent,        an alkylating agent of this salt formed of an alkylaluminium, and        a halide of an alkylaluminium.        
For example, the document “Report of the Academy of Sciences of the U.S.S.R., volume 234, No. 5, 1977 (Y. B. Monakov, Y. R. Bieshev, A. A. Berg, S. R. Rafikov)” shows that isoprene may be polymerized at a temperature of between 20° C. and 50° C. using a catalytic system comprising:                a bis(2-ethylhexyl)phosphoric acid salt of neodymium or praseodymium, as rare earth salt, in solution in toluene,        triisobutylaluminium as alkylating agent, in a molar ratio (alkylating agent/rare earth salt) of 20, and        diethylaluminium chloride as halide of an alkylaluminium.        
Mention may also be made of the document “Proceedings of China—U.S. Bilateral Symposium on Polymer Chemistry and Physics, Science Press, pp. 382–398, 1981 (O. Jun, W. Fosong, S. Zhiquan)”. This document teaches the use of a bis(2-ethylhexyl)phosphoric acid salt of neodymium, in association with triethylaluminium or triisobutylaluminium, and an alkylaluminium halide of the formula Al2(C2H5)3Cl3. The polyisoprenes which are obtained by means of such a catalytic system have a cis-1,4 linkage content which varies from 94.2% to 94.7% (see Tables 4 and 6, pp. 386 and 387).
This document also mentions the use of catalytic systems based on:                rare earth naphthenate, wherein the cis-1,4 linkage content of the obtained polyisoprenes is between 93.6% and 96.0%; and        rare earth trichloride (catalytic system of formula LnCl3-C2H5OH—Al(C2H5)3), wherein the cis-1,4 linkage content of the obtained polyisoprenes being between 94.1% and 98.0% (this content of 98% being achieved using ytterbium as rare earth, see Table 12 p. 391).        
In the majority of cases, the microstructure of the polyisoprene is determined by the technique of medium-wave infrared radiation analysis (abbreviated to MIR) in accordance with the method developed by Ciampelli et al (F. Ciampelli, D. Moreno, M. Cambini, Makromol. Chem., 1963, 61, 250–253). It will be noted that this method, which is based solely on calculations made in the infrared range, does not always provide results of satisfactory accuracy when used in isolation.
U.S. Pat. No. 5,859,156 describes a process for the preparation of polyisoprenes by means of a catalytic system based on titanium tetrachloride, an organoaluminium and an ether. The maximum cis-1,4 linkage content of the polyisoprenes obtained, measured by carbon-13 nuclear magnetic resonance (13C-NMR), was 98.0% (see example 2, column 27, the content of trans-1,4 and 3,4 linkages then being 1.0% each).